Edition explicitly says:

and:

As HTML (before HTML5) is based on SGML, its parsing also depends on the Document Type Definition (DTD), specifically an HTML DTD (e.g. HTML 4.01). The DTD specifies which element types are possible (i.e. it defines the set of element types) and also the valid combinations in which they may appear in a document. It is part of general SGML behavior that, where only one valid structure is possible

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